I. Field of the Invention
The present invention relates generally to the transmission of high rate digital signals such as high definition television (HDTV) signals, and more particularly to orthogonal frequency division multiplexing (OFDM) systems that are used in the transmission of digital signals.
II. Description of the Related Art
Orthogonal frequency division multiplexing (OFDM) is a technique for broadcasting high rate digital signals such as high definition television (HDTV) signals. In OFDM systems, a single high rate data stream is divided into several parallel low rate substreams, with each substream being used to modulate a respective subcarrier frequency. It should be noted that although the present invention is described in terms of quadrature amplitude modulation, it is equally applicable to phase shift keyed modulation systems.
The modulation technique used in OFDM systems is referred to as quadrature amplitude modulation (QAM), in which both the phase and the amplitude of the carrier frequency are modulated. In QAM modulation, complex QAM symbols are generated from plural data bits, with each symbol including a real number term and an imaginary number term and with each symbol representing the plural data bits from which it was generated. A plurality of QAM bits are transmitted together in a pattern that can be graphically represented by a complex plane. Typically, the pattern is referred to as a xe2x80x9cconstellationxe2x80x9d. By using QAM modulation, an OFDM system can improve its efficiency.
It happens that when a signal is broadcast, it can propagate to a receiver by more than one path. For example, a signal from a single transmitter can propagate along a straight line to a receiver, and it can also be reflected off of physical objects to propagate along a different path to the receiver. Moreover, it happens that when a system uses a so-called xe2x80x9ccellularxe2x80x9d broadcasting technique to increase spectral efficiency, a signal intended for a received might be broadcast by more than one transmitter. Hence, the same signal will be transmitted to the receiver along more than one path. Such parallel propagation of signals, whether man-made (i.e., caused by broadcasting the same signal from more than one transmitter) or natural (i.e., caused by echoes) is referred to as xe2x80x9cmultipathxe2x80x9d. It can be readily appreciated that while cellular digital broadcasting is spectrally efficient, provisions must be made to effectively address multipath considerations.
Fortunately, OFDM systems that use QAM modulation are more effective in the presence of multipath conditions (which, as stated above, must arise when cellular broadcasting techniques are used) than are QAM modulation techniques in which only a single carrier frequency is used. More particularly, in single carrier QAM systems, a complex equalizer must be used to equalize channels that have echoes as strong as the primary path, and such equalization is difficult to execute. In contrast, in OFDM systems the need for complex equalizers can be eliminated altogether simply by inserting a guard interval of appropriate length at the beginning of each symbol. Accordingly, OFDM systems that use QAM modulation are preferred when multipath conditions are expected.
With particular regard to current OFDM systems to understand why the present invention is useful and needed, in current systems the data stream to be broadcast is encoded twice, first with a Reed-Solomon encoder and then with a trellis coding scheme. It should be noted that the present invention is equally applicable to systems in which there is only one coding. In a typical trellis coding scheme, the data stream is encoded with a convolutional encoder and then successive bits are combined in a bit group that will become a QAM symbol. Several bits are in a group, with the number of bits per group being defined by an integer xe2x80x9cmxe2x80x9d (hence, each group is referred to as having an xe2x80x9cm-aryxe2x80x9d dimension). Typically, the value of xe2x80x9cmxe2x80x9d is four, five, six, or seven, although it can be more or less.
After grouping the bits into multi-bit symbols, the symbols are interleaved. By xe2x80x9cinterleavingxe2x80x9d is meant that the symbol stream is rearranged in sequence, to thereby randomize potential errors caused by channel degradation. To illustrate, suppose five words are to be transmitted. If, during transmission of a non-interleaved signal, a temporary channel disturbance occurs. Under these circumstances, an entire word can be lost before the channel disturbance abates, and it can be difficult if not impossible to know what information had been conveyed by the lost word.
In contrast, if the letters of the five words are sequentially rearranged (i.e., xe2x80x9cinterleavedxe2x80x9d) prior to transmission and a channel disturbance occurs, several letters might be lost, perhaps one letter per word. Upon decoding the rearranged letters, however, all five words would appear, albeit with several of the words missing letters. It will be readily appreciated that under these circumstances, it would be relatively easy for a digital decoder to recover the data substantially in its entirety. After interleaving the m-ary symbols, the symbols are mapped to complex symbols using QAM principles noted above, multiplexed into their respective sub-carrier channels, and transmitted.
As recognized by the present invention, however, current OFDM systems that use the above-mentioned trellis coding scheme, in which data bits are grouped into symbols prior to interleaving, exhibit performance shortcomings in the presence of multipath conditions in which some of the OFDM sub-carriers are severely attenuated. As further recognized herein, it is possible to improve the performance of OFDM systems in the presence of sub-carrier attenuation caused by multipath conditions. As still further recognized by the present invention, the performance of such an OFDM system can be further improved by undertaking soft decision-making at the receiver in determining received data values.
Accordingly, it is an object of the present invention to provide a system for transmitting high rate digital data in the presence of multipath conditions. Another object of the present invention is to provide a system for transmitting high rate digital data using OFDM principles, which performs comparatively effectively in the presence of sub-carrier attenuation in multipath conditions. Still another object of the present invention is to provide a system for receiving high rate digital data which permits the use of soft decision making on a sub-channel by sub-channel basis to determine data values. Yet another object of the present invention is to provide a system for transmitting high rate digital data that is easy to use and cost-effective to manufacture and implement.
In an orthogonal frequency division multiplexing (OFDM) transmitter, a device is disclosed for processing digital data bits for transmission thereof to a receiver. The device includes an outer interleaver, preferably a Reed-Solomon code symbol interleaver, for processing the data bits and an inner interleaver for receiving the processed output data bits from the outer interleaver and interleaving the data bits. Also, the device includes means for receiving the interleaved data bits from the inner interleaver and generating a symbol representative of xe2x80x9cmxe2x80x9d successive bits from the inner interleaver, wherein xe2x80x9cmxe2x80x9d is an integer greater than one.
In the preferred embodiment, a convolutional encoder processes bits between the inner and outer interleavers. Moreover, a means is provided for mapping each symbol to m-ary signal space. As intended by the preferred embodiment, the mapping means uses quadrature amplitude modulation (QAM) to thereby generate complex symbols. In the case wherein xe2x80x9cmxe2x80x9d is an odd integer at least equal to five (5), the mapping means minimizes the sum of the Hamming distances between neighboring symbols in a quadrant of the signal space.
As disclosed in further detail below, a serial to parallel converter processes the complex symbols into xe2x80x9cnxe2x80x9d substreams, wherein xe2x80x9cnxe2x80x9d is an integer greater than one. A guard period generator establishes a guard period in the signal streams. The device is disclosed in combination with the OFDM transmitter, and in further combination with an OFDM system.
In another aspect, a method for transmitting digital data bits using orthogonal frequency division multiplexing (OFDM) includes convolutionally encoding the bits, then interleaving the bits. Next, the method includes grouping xe2x80x9cmxe2x80x9d bits in parallel to establish a respective symbol.
In yet another aspect, for a receiver receiving xe2x80x9cnxe2x80x9d substreams of an orthogonal frequency division multiplexed (OFDM) signal containing complex phase-adjusted symbols, wherein each symbol represents xe2x80x9cmxe2x80x9d data bits, a device is disclosed which includes, for each substream, a soft decision quantizer for determining a binary value of each bit represented by each symbol in the substream. A computer logic device is also disclosed for undertaking this part of the receiver function.
In still another aspect, a device is disclosed for transmitting orthogonal frequency division multiplexing (OFDM) signals. The device includes quadrature amplitude modulation (QAM) means for generating a plurality of QAM symbols. Also, the device includes mapping means for mapping the symbols to m-ary space such that the sum of the Hamming distances between neighboring symbols in the space is minimized, wherein m is an odd integer at least equal to five (5).
The details of the present invention, both as to its structure and operation, can best be understood in reference to the accompanying drawings, in which like reference numerals refer to like parts, and in which: